1. Field of the Invention
The present invention relates to a method for designing multi-threshold complementary metal-oxide-semiconductor (MTCMOS) circuits and the physical architecture of the circuits resulting from using such a method. In particular, the present invention relates to a design method for providing power gating in an MTCMOS circuit using standard cells that provide a “context-sensitive” conductive traces, and the physical circuit architecture of MTCMOS circuits resulting from using this method.
2. Discussion of the Related Art
A significant concern in integrated circuit design is reducing leakage currents. Leakage currents flow in logic circuits from a power supply node into the ground node because the switching characteristics of the transistors in the logic circuits are not ideal (i.e., the transistors cannot be completely shut off).
In MTCMOS circuits, one technique that reduces leakage current is to a place a “power gate” (also known as “power switch” or simply, “switch cell”) between the lowest potential terminal of a logic gate (the “virtual ground” reference) and the ground reference. This technique is illustrated schematically in FIG. 1, which shows power gate or switch cell 101 controlling the leakage current path of logic cell 102 to ground. As shown in FIG. 1, logic cell 102 is formed using lower threshold voltage transistors to provide short switching times. The power gate is typically a transistor which has a higher threshold voltage than the threshold voltage of the transistors used to implement the logic cells. The power gate interrupts the leakage current path to ground. When power gate 101 is conducting (i.e., a high voltage is provided at control node 106), a leakage current flows from power supply node 104 through logic cell 102 to virtual ground node 103, and through power gate 101 to true ground node 105. However, during standby (i.e., when a voltage much less than power gate 101's threshold voltage is imposed at control node 106), power gate 101 cuts off the leakage current path from virtual ground node 103 to true ground node 105.
Several design methods have been used to provide power gate cells. One method (“integrated switches”), as illustrated by way of example in FIG. 2, integrates power gate 202 with logic cell 201. In this arrangement, logic cells, such as logic cell 201, are placed in rows according to a conventional standard cell design method. As shown in FIG. 2, conductors 204a and 204b are part of a power supply grid providing a power supply voltage to the logic cells. Similarly, conductors 203a and 203b are part of a ground reference grid providing a true ground reference to the logic cells. Virtual ground nodes are located within each logic cell (e.g., logic cell 201).
FIG. 3 illustrates a second method (“cavity switches”) for placing power gates, in which a row of logic cells (e.g., logic cells 301a, 301b, 301c and 301d) share power switches provided in an adjacent dedicated row (“switch cavity area”; e.g., power switch cavity area 302). Conductors 303a and 303b provide the true ground voltage reference, and conductors 304a and 304b provide the power supply voltage reference.
FIG. 4 illustrates a third method (“ring switches”) for placing power gates, in which a group of logic cells (e.g., logic cell 401) share power switches placed in an annular strip (e.g., power gate area 402) encircling the logic cells. Power switches within power gate area 402 are typically connected in parallel. Conductors outside power gate area 402 route the true ground to the power gates in power gate area 402, Virtual ground reference nodes are provided in the area between power gate area 402 and logic cells 401.
FIG. 5 illustrates a fourth method (“grid switches”) for placing power gates, in which power switches (e.g., power switches in power switch areas 502a and 502b) are placed in predetermined locations at regular intervals to service rows of logic cells in their proximity (e.g., logic cells 502a and 501b are serviced by power switches in power switch 502a). The power switches in the predetermined power switch areas may be connected in parallel, and conductors (e.g., conductors 505a, 505b and 505c) running orthogonal to the rows of logic cells may provide a virtual ground reference grid.
The grid switches method, as illustrated in FIG. 5, requires standard cells that include the virtual ground rail (Vssv) as a third supply rail, in addition to the usual power (Vdd) and ground (Vss) rails. This method is further illustrated in FIG. 6. In FIG. 6, standard cells 621 and 622 are connected to virtual ground wire 623 (running horizontally), which is connected by switch cell 626 to ground wire 624. Virtual ground wire 623 is connected to other virtual ground wires by virtual ground bus 625 (running vertically). A considerable silicon area cost is therefore incurred in providing the third rail.
In a standard cell of the prior art, one or more well taps (i.e., each a connection to the P-well or N-well substrate of the PMOS or NMOS transistors in the standard cell) are provided within the standard cell, so that a high resistance does not develop between the substrate of the transistors in the standard cell and the power or ground reference.